This invention relates to a process for inhibiting corrosion during acid pickling operations employed in the treatment of iron-containing metals, particularly steel.
During the processing of steel to form steel sheets, plates, tubes, nails, wire, and other articles, the steel is subjected to elevated temperatures which results in the formation of iron oxide. Before the steel can be subjected to further processing, such as electroplating, cold rolling and galvanizing, the iron oxide must be removed. This is typically done by immersing the sheets, plates, tubes, nails and so on in a suitable acid solution. Hydrochloric acid is frequently used in this application. In this process, typically called acid pickling or steel pickling, the acid solution attacks and dissolves the iron oxide. This process may be represented by the equation EQU FeO+2HCl.fwdarw.FeCl.sub.2 +H.sub.2 O.
Unfortunately, the acid solution will also attack the steel itself once the iron oxide layer is removed. This second reaction is represented by the equation EQU Fe+2HCl.fwdarw.FeCl.sub.2 +H.sub.2.
This second reaction can lead to the loss of a substantial amount of steel during the pickling process. Approximately 0.5 percent of the steel processed may be dissolved which has a significant economic impact. When a typical plant might treat approximately a million tons of steel per year, the loss of even 0.5 percent of the steel treated would be the loss of 5000 tons of steel. At a price of three hundred dollars per ton, this loss could represent about 1.5 million dollars per year. For these reasons, it is desirable to use a corrosion inhibitor during the pickling of steel.
A desirable corrosion inhibitor is one which enhances the selectivity of the acid toward the surface impurities on the metal, decreases the selectivity of the acid toward the metal itself, is economical to use, and is environmentally safe. Various compositions have been identified as corrosion inhibitors. U.S. Pat. Nos. 3,607,781 and 3,676,354 describe a corrosion inhibitor comprising a surface-active agent, a hydroxyalkylamine and a combination of mineral acids. The use of a mixture of higher primary aliphatic and alicyclic amines with water-soluble, nonionic emulsifying agents as a corrosion inhibitor is disclosed in U.S. Pat. No. 2,649,415. Polyglycolamines have also been disclosed as corrosion inhibitors in U.S. Pat. No. 4,171,279. However, these compositions and others known in the art are often complex, sometimes environmentally objectionable, sometimes not effective under normal use conditions and frequently expensive. As a consequence, existing corrosion inhibitors do not meet the needs of many steel manufacturers. Thus, some steel manufacturers use no corrosion inhibitor at all.
What is needed is a corrosion inhibitor which is effective, simple, environmentally unobjectionable and relatively inexpensive.